A magnetic sensor which uses a magnetoresistance effect element is useful for detecting a change of a physical amount in a non-contacting manner. In a magnetic sensor which detects a rotation angle (hereinafter referred to as a rotation angle sensor), a superior detection sensitivity of the magnetoresistance effect element with respect to a rotational magnetic field is required.
As a film used for the magnetoresistance effect element, there exist an anisotropic magnetoresistance effect (AMR) film, a coupling giant magnetoresistance effect (GMR) film, a spin valve giant magnetoresistance effect (SVGMR) film, etc.
A magnetoresistance effect element which uses the AMR film is normally formed by patterning a single layer film of a NiFe alloy thin film or the like. Although the process is simple, the change in resistance of the element of only approximately 3% can be obtained.
On the other hand, Patent Literature 1 shows a magnetoresistance effect element which uses the GMR film. Unlike the magnetoresistance effect element which uses the AMR film, the magnetoresistance effect element of Patent Literature 1 uses an artificial lattice metal film in which a few tens of layers of NiCoFe alloy thin films and non-magnetic metal thin films are alternately layered. This magnetoresistance effect element can achieve a large change in resistance which is 2-4 times that of the magnetoresistance effect element which uses the AMR film.
Patent Literature 2 discloses a magnetoresistance effect element which uses the SVGMR film (a replay head of a magnetic disk device). The SVGMR film used in the magnetoresistance effect element of Patent Literature 2 comprises a pinned layer, a free layer, and a non-magnetic layer. The pinned layer is configured such that a magnetization direction is not changed even when the direction of magnetic field (magnetic flux) changes. The free layer is configured such that the magnetization direction changes following the change of the magnetic field. The non-magnetic layer magnetically separates the pinned layer and the free layer. The resistance is minimized when the magnetization directions of the pinned layer and the free layer are parallel to each other, and the resistance is maximized when the magnetization directions are anti-parallel to each other. The magnetoresistance effect element using the SVGMR film shows a high change in resistance of greater than or equal to that of the magnetoresistance effect element using the GMR film. The magnetoresistance effect element using the SVGMR film shows a change in resistance of greater than or equal to 7% with a very weak magnetic field (0.8˜2 kA/m (approximately 10˜20 Oe)), and can be used for a magnetic sensor which requires a high detection sensitivity.
As the SVGMR film, a structure is widely known with an antiferromagnetic layer/a ferromagnetic layer/an intermediate layer/a free layer/a protection layer (hereinafter referred to as “antiferromagnetic SVGMR film”). The antiferromagnetic layer is formed using an antiferromagnetic material, and applies a unidirectional anisotropy to the magnetization of the pinned layer. On the other hand, there also is known a structure in which the antiferromagnetic layer is not provided and the pinned layer is formed in a structure of a first ferromagnetic layer/an antiferromagnetic coupling layer/a second ferromagnetic layer (hereinafter referred to as a “self-pinned SVGMR film”). Magnetoresistance effect elements using such a self-pinned SVGMR film are disclosed in Patent Literature 3, Patent Literature 4, and Non-Patent Literature 1.
In order to create a rotation angle sensor which detects angles of 360 degrees using the SVGMR film, magnetizations of the pinned layers in a plurality of directions are necessary. Because of this, when a rotation angle sensor is to be created using the antiferromagnetic SVGMR film which is unidirectionally anisotropic, a plurality of sensor elements which are unidirectionally anisotropic must be used, and thus difficulty arises in integrating the sensor elements.
On the other hand, because the self-pinned SVGMR film does not have the antiferromagnetic layer, a rotation angle sensor in which anisotropies in 4 directions are applied in one sensor element can be created by layering self-pinned SVGMR films over a same substrate, and the integration of the sensor elements can be easily achieved.
In addition, there are a high-temperature limit temperature known as Néel temperature for the antiferromagnetic layer of the antiferromagnetic SVGMR film and a high-temperature limit temperature known as a blocking temperature for an exchange coupling force of the pinned layer, and when this temperature is reached, the exchange coupling force substantially disappears. In addition, because the exchange coupling force is reduced as the temperature becomes close to the blocking temperature, the exchange coupling force may become insufficient even when the temperature is lower than the blocking temperature, resulting a difficulty in achieving a sufficient precision for the rotation angle sensor using the antiferromagnetic SVGMR film under a high temperature.